Lecture 18: Animal Structure and Function

1) organisms have the capacity to adjust to environmental change over the long term and short term by physiological responses (these traits evolved).

• Over the long term, morphological traits have evolved; for example, the long, tonguelike proboscis of a hawkmoth is a structural adaptation for feeding.

• For a physiological example, while most insects are inactive when cold, the hawkmoth, Manduca sexta, can forage for nectar when air temperatures are as low as 5oC.

• The moth uses a shivering-like mechanism for preflight warm up of its flight muscles.

• Once in flight, the waste heat of metabolic activity in the flight muscles and other adaptations maintain a muscle temperature of 30oC, even when the external environment is close to freezing.

2) Tissues are groups of cell with a common structure and function.

• Different types of tissues have different structures that are especially suited to their functions.

• A tissue may be held together by a sticky extracellular matrix that coats the cells or weaves them together in a fabric of fibers.

• The term tissue is from a Latin word meaning “weave.”

• Tissues are classified into four main categories: epithelial tissue, connective tissue, nervous tissue, and muscle tissue.

• Epithelia are classified by the number of cell layers and the shape of the cells on the free surface.

• A simple epithelium has a single layer of cells, and a stratified epithelium has multiple tiers of cells.

• The shapes of cells may be cuboidal (like dice), columnar (like bricks on end), or squamous (flat like floor tiles).

• The major types of connective tissues in vertebrates are loose connective tissue, adipose tissue, fibrous connective tissue, cartilage, bone, and blood.

• Adipose tissue is a specialized form of loose connective tissues that store fat in adipose cells distributed throughout the matrix.

• Adipose tissue pads and insulates the body and stores fuel as fat molecules.

• Each adipose cell contains a large fat droplet that swells when fat is stored and shrinks when the body uses fat as fuel.

• Blood functions differently from other connective tissues, but it does have an extensive extracellular matrix.

• Muscle tissue is composed of long cells called muscle fibers that are capable of contracting when stimulated by nerve impulses.

3) Homeostasis

• In a negative-feedback system, a change in the variable being monitored triggers the control mechanism to counteract further change in the same direction.

• Owing to a time lag between receptor and response, the variable drifts slightly above and below the set point, but the fluctuations are moderate.

• Negative-feedback mechanisms prevent small changes from becoming too large.

• Most homeostatic mechanisms in animals operate on this principle of negative feedback.

• In contrast to negative feedback, positive feedback involves a change in some variable that trigger mechanisms that amplify rather than reverse the change.

• For example, during childbirth, the pressure of the baby’s head against sensors near the opening of the uterus stimulates uterine contractions.

• These cause greater pressure against the uterine opening, heightening the contractions, which cause still greater pressure.

• Positive feedback brings childbirth to completion, a very different sort of process from maintaining a steady state.

• Over the short term, homeostatic mechanisms can keep a process, such a body temperature, close to a set point, whatever it is at that particular time.

• But over the longer term, homeostasis allows regulated change in the body’s internal environment.

• Internal regulation is expensive and animals use a considerable portion of their energy from the food they eat to maintain favorable internal conditions.

4) Energetics

• Food is digested by enzymatic hydrolysis, and energy-containing food molecules are absorbed by body cells.
• Most fuel molecules are used to generate ATP by the catabolic processes of cellular respiration and fermentation.
• The chemical energy of ATP powers cellular work, enabling cells, organs, and organ systems to perform the many functions that keep an animal alive.
• Since the production and use of ATP generates heat, an animal must continuously loose heat to its surroundings.
• After basic energetic needs are met, any remaining food molecules can be used
  in biosynthesis.
• This includes body growth and repair, synthesis of storage material such as fat, and production of
  reproductive structures, including gametes.
• Biosynthesis requires both carbon skeletons for new structures and ATP to power their assembly.
• The amount of energy an animal uses in a unit of time is called its metabolic rate - the sum of all the energy-requiring biochemical reactions occurring over a given time interval.
• Energy is measured in calories (cal) or kilocalories (kcal).
• A kilocalorie is 1,000 calories.
• The term Calorie, with a capital C, as used by many nutritionists, is actually a kilocalorie.
• Metabolic rate can be determined several ways.
• Because nearly all the chemical energy used in cellular respiration eventually appears as heat, metabolic rate can be measured by monitoring an animal’s heat loss.
• A small animal can be placed in a calorimeter, which is a closed insulated chamber equipped with a device that records the animals heat loss.
• A more indirect way to measure metabolic rate is to determine the amount of oxygen consumed or carbon dioxide produced by an animal’s cellular respiration.
• There are two basic bioenergetic “strategies” used by animals.
• Birds and mammals are mainly endothermic, maintaining their body temperature at a certain level with heat generated by metabolism.
• Endothermy is a high-energy strategy that permits intense, long-duration activity of a wide range of environmental temperatures.
• Most fishes, amphibians, reptiles, and invertebrates are ectothermic, meaning they do not produce enough metabolic heat to have much effect on body temperature.
• The ectothermic strategy requires much less energy than is needed by endotherms, because of the energy cost of heating (or cooling) an endothermic body.
• However, ectotherms are generally incapable of intense activity over long periods.
• One of animal biology’s most intriguing, but largely unanswered questions has to do with the relationship between body size and metabolic rate.
• Physiologists have shown that the amount of energy it takes to maintain each gram of body weight is inversely related to body size.
• For example, each gram of a mouse consumes about 20 times more calories than a gram of an elephant.
• The higher metabolic rate of a smaller animal demands a proportionately greater delivery rate of oxygen.
• A smaller animal also has a higher breathing rate, blood volume (relative to size), and heart rate (pulse) and must eat much more food per unit of body mass.
• Every animal has a range of metabolic rates.
• Minimal rates power the basic functions that support life, such as cell maintenance, breathing, and heartbeat.
• The metabolic rate of a nongrowing endotherm at rest, with an empty stomach, and experiencing no stress is called the basal metabolic rate (BMR).
• The BMR for humans averages abut 1,600 to 1,800 kcal per day for adult males and about 1,300 to 1,500 kcal per day for adult females.
• In ectotherms, body temperature changes with temperature of the surroundings, and so does metabolic rate.
• Therefore, the minimal metabolic rate of an ectotherm must be determined at a specific temperature.
• The metabolic rate of a resting, fasting, nonstressed ectotherm is called its standard metabolic rate (SMR).
• For both ectotherms and endotherms, activity has a large effect on metabolic rate.
• Any behavior consumes energy beyond the BMR or SMR.
• Maximal metabolic rates (the highest rates of ATP utilization) occur during peak activity, such as lifting heavy weights, all-out running, or high-speed swimming.
• Different species of animals use the energy and materials in food in different ways, depending on their environment, behavior, size, and basic energy “strategy” of endothermy or ectothermy.
• For most animals, the majority of food is devoted to the production of ATP, and relatively little goes to growth or reproduction.
• However, the amount of energy used for BMR (or SMR), activity, and temperature control varies considerably between species.
• For example, the typical annual energy budget of four vertebrates reinforces two important concepts in bioenergetics.
• First, a small animal has a much greater energy demand per kg than does a large animal of the same class.
• Second, an ectotherm requires much less energy per kg than does an endotherm of equivalent size.